Wireless automatic meter reading systems are well known. Typically, each utility meter is provided with some type of encoder/receiver/transmitter endpoint, such as the Itron, Inc. ERT. In the instance of gas and water meters, that endpoint is likely battery-powered. The endpoint collects the meter readings and periodically transmits those readings over a network to a central station. The power limitations imposed by the need for the endpoint to be battery powered and by regulations governing radio transmissions often prevent direct radio transmissions to the central station. Instead, wireless meter reading systems utilize a layered network of overlapping intermediate receiving stations that receive transmissions from a group of meter encoders and forward those messages on to the next higher layer in the network as described, for example, in U.S. Pat. No. 5,056,107, which is hereby incorporated by reference. These types of layered wireless transmission networks allow for the use of lower power, unlicensed wireless transmitters in the thousands of endpoints that must be deployed as part of a utility meter reading system for a large metropolitan area.
In 1985, as an attempt to stimulate the production and use of wireless network products, the FCC modified Part 15 of the Radio Spectrum Regulation, which governs unlicensed devices. The modification authorized wireless network products to operate in the industrial, scientific, and medical (ISM) bands using spread spectrum modulation. The ISM frequencies that may be used include 902 to 928 MHz, 2.4 to 2.4835 GHz, and 5.725 to 5.850 GHz. The FCC allows users to operate spread spectrum wireless products, such as utility metering systems, without obtaining FCC licenses if the products meet certain requirements. This deregulation of the frequency spectrum eliminates the need for the user organizations to perform costly and time-consuming frequency planning to coordinate radio installations that will avoid interference with existing radio systems.
Currently, synchronization between a transmitter and a receiver is accomplished by using a synchronization pulse, which may interfere with hardware clock recovery, or by using Barker coding type approaches, which require receivers to stay on at length. However, these current synchronization techniques do not prevent collisions in endbound data packets as the endpoints are not time synchronized between the transmitter and receiver. This lack of endpoint synchronization further results in limitations on individual cell capacities as well as inherent difficulties in implementing message integration techniques.
Accordingly, there is a need for a battery efficient system for time synchronizing messages between transmitter and receiver that not only increases individual cell capacities but also allows for the implementation of two-way message integration techniques.